It is desirable to reduce as much as possible the amount of light reflected at the air-glass interface of an optical element e.g. an optical lens, to increase the light throughput and, in some instances, to minimize ghost images. In some applications, the coating must perform over a wide wavelength range.
Several types of antireflective coatings are known already. U.S. Pat. No. 5,572,086 to Tong et al. proposes an antireflective and antistatic coating for a CRT display panel. The multi-layer coating comprises a first conductive grounded inner coating including a metal salt such as antimony-tin oxide, and an outer coating disposed on the first coating and comprising a water soluble organic or inorganic salt or a polymer soluble in an organic solvent. The outer coating has a plurality of pores of various depths for providing the outer coating with a range of light refractive indexes determined by the depth of the pores.
Boulos et al, U.S. Pat. No. 5,208,181 proposes an antireflective coating having a metal oxide and a graded concentration of light metal fluoride. The process for forming the coatings is also described in Boulos et al, U.S. Pat. No. 5,268,196.
Both these patents describe an approach in which the fluorination of the fluoride layer is controlled such that the concentration of the fluoride varies across the thickness of the fluoride layer.
There is still a need for efficient, durable, easily-applied and relatively inexpensive antireflective (AR) coatings having a passband from about 360 to about 1400 nanometers.